There is no argument against it that isn't motivated by greed. A single auction house would benefit every player except the extremely minute minority who enjoy creating monopolies and controlling entire markets. As someone who's participated in that part of the game, I completely understand how fun it is and I sympathize with those players when they argue against CRAH, however this is simply a case where the benefit to the masses just far outweighs their needs/wants.

Edit: Also note that a poll in this particular subforum will be significantly skewed against CRAH simply because this forum attracts a disproportionate number of players who spend a lot of time controlling markets.

Here's one for you: It would make it much much harder to use auction tools to find out what your item is worth. Each realm already has between 20-60k auctions. If that were merged, scanning the AH with a tool such as Auctioneer would become nigh impossible.

If you think using Auctioneer is being "motivated by greed", then I postulate one further: Selling using the AH is motivated by greed. Otherwise, why not just vendor the items? QED.

It would be a disaster introducing CRAH. If anyone here has studied economics, it is quite obvious that it will not work. Each server has different levels of inflation but each server also has the same currency, i.e. gold. The realm with high inflation (i.e. high price levels) would most likely hold more gold. Merging realms together would most likely mean highly inflated realms would reap the benefits in the short run at least. There is a reason why Blizz has limited gold transfer when moving realms and that is to loosely control for inflation. At the end of the day, differences in inflation would make this task almost impossible unless Blizz is able to match inflation between realms.